Three months ago, Cindy Colvin could barely walk. Shopping sprees to feed her "first addiction, retail" were out of the question, as were the essentials — getting groceries, tending to household chores and making it into work.

"I didn't know how to walk anymore," said Colvin, 55. "I wasn't walking like a normal person. Just to take a step, every single step, was agony."

With knees so swollen she "couldn't remember normal" and a constant throbbing throughout both legs, the Avondale, Pa., resident drove an hour and a half to Baltimore to see OrthoMaryland's Dr. Barry Waldman, who promised a new life made possible by 3D printing.

Waldman suggested a modern approach to total knee replacement using 3D printing technology to cast an implant and manufacture the jigs — plastic cutting guides — that direct the surgeon's incisions.

The procedure taps into a ballyhooed technology that is rapidly gaining economic significance as practical uses are identified. The market for 3D printing, estimated at $3.8 billion today by Canalys, is expected to grow to $16.2 billion by 2018. In medicine, uses such as crafting artificial skin grafts and creating synthetic heart valves are prompting some doctors to re-think tried-and-true medical procedures.

"The potential it has to change lives is just incredible," said Joe Kempton, a Canalys analyst who covers 3D printing.

Colvin said her April 15 complete double-knee replacement surgery performed by Waldman at Sinai Hospital of Baltimore gave her her life back, saying her legs are "stronger than they've ever been."

"The whole time before, I just knew I had to think forward, to when my knees do bend and move normally again," she said. "The day that they did, the day it stopped hurting — I can't even describe it. Being able to just move again was incredible."

The former multi-sport athlete hobbled around on increasingly bowlegged legs, bent 6 degrees out of alignment, for more than 10 years, until she "just couldn't take the pain anymore." One CT scan, two hours of surgery and nine weeks later, Colvin is discovering a new spring to her step.

"It affected every single decision I made until the surgery," she said. "Going forward knowing you can't exercise, can't do any kind of exercise anymore — it was awful."

Unlike traditional knee replacements that employ implants in a range of sizes — much like shoes — Waldman promised her a fit all her own made possible by a "3D map" of the knee, hip and ankle created from a CT scan. The body's blueprint is then used to create the replacement joints from cobalt chrome cast in wax models that are created with a 3D printer one sub-millimeter layer at a time.

Waldman has performed 100 knee replacements with the technology, which he credits for reducing the amount of bone he has to shave off to install each implant. Six four-inch custom jigs replace a two-foot metal rod that had to be adjusted inside the thigh bone to guide incisions — a process Waldman said took 15 minutes longer, causing the patient to lose more blood during surgery.

"You can take an off-the-shelf implant, and it may not fit," Waldman said. "Plenty of times we'd be in the middle of surgery and say this is between a '5' and a '6,' and we need a '5.5.' Well, there is no '5.5'"

Boston-based ConforMIS created Colvin's customized metal implant, as well as five 3D-printed plastic jigs used to guide Waldman's incisions. The disposable jigs are custom-made for each surgery, replacing up to 50 differently sized instruments that had to be sterilized after each use.

But some surgeons remain wary of the new procedure, especially since the ConforMIS implants were just approved by the FDA in 2011.

Dr. Tariq Nayfeh, chief of the Center for Joint replacement at MedStar Harbor Hospital, said while the technology "certainly shows promise," it's hard to justify the added cost of a CT scan, as well as the ensuing radiation exposure.

While "ConforMIS does make the knee custom to the patient … there's next to zero track record," Nayfeh said. "Meanwhile, there's 25-year track records on the other things that are used. There's absolutely nothing to demonstrate that this is better. It's a good investigative thing to look into."

Dr. Scott Rosenfeld, a pediatric orthopedic surgeon at Texas Children's Hospital, said off-the-shelf implants are fine for the average 50-to-60-year-old adult, but he sees the benefit in custom implants for young patients and those with skeletal deformities. He said he would consider using custom implants on a "case-by-case" basis, but hasn't done so yet.

Colvin credits the procedure for a speedy recovery that had her ambling around with the aid of a walker the day after surgery. A little more than two months later, she has walked around New York City visiting her son, Bryan — whom she hadn't been able to see for more than a year. She's also logged miles on a recumbent bike and started swimming again.

In 10 years, Colvin said, she expects to be capable of 99 percent of what she once was — tennis to water skiing and everything in between.

"I see myself way more active," she said, tracing her fingers over her scars, "being able to be part of raising my grandchild, being able to be in my children's lives, being able to do everything I missed so much for so long."